This invention relates to the measurement of fluid flow and, more particularly, to the determination of the flow characteristic of a flowmeter.
In order to obtain accurate readings from a flowmeter, it must be calibrated from time to time by determining its characteristic, i.e., the constant of proportionality between the flow rate of the fluid flowing through the flowmeter and the resposne given by the flowmeter, sometimes called the K-factor of the flowmeter. In the case of a turbine type flowmeter that develops electrical oscillations proportional in number to the volume of flow through the flowmeter, this characteristic is expressed in terms of the number of pulses generated by the flowmeter per unit volume of fluid passing there through. The flowmeter characteristic is a function of the type of fluid, as well as the fluid temperature, pressure, and flow rate, and varies as the parts of the flowmeter wear in the course of use. Apparatus to determine the characteristic of a flowmeter while in an operating fluid system is called a prover. Apparatus to determine the characteristic of a flowmeter in a self-contained system, i.e., not in an operating fluid system, is called a calibrator.
My U.S. Pat. No. 4,152,922 discloses a small-volume prover that employs mechanical volume displacement techniques. The prover has a measuring piston that travels through a measuring cylinder as a fluid barrier in synchronism with fluid passing through the operating fluid system that includes the flowmeter under test. A rod connects the measuring piston to a fluidically actuated control piston in a control cylinder which serves to hold the measuring piston at the upstream end of the measuring cylinder between test runs and return the measuring piston to the upstream end of the measuring cylinder after each test run. When the measuring piston is released at the upstream end of the measuring cylinder to start a test run, the momentum of the fluid flowing through the system rapidly accelerates the measuring piston to the same speed as the fluid flowing through the measuring cylinder, which is representative of the flow rate passing through the flowmeter. The K-factor is determined by counting the number of pulses produced by the flowmeter during the time interval of a given volumetric displacement of the measuring piston.
Although the state-of-the-art of low-volume mechanical displacement provers has rapidly advanced in recent years, the development of small-volume calibrators has not kept pace. The large fluid volume requirements often make it impractical to duplicate the actual conditions, i.e., pressure, temperature, and fluid type, in the operating system of the flowmeter, so the K-factor must be derived inferentially. In small-volume operation, drag represented by the friction of the measuring piston and leakage across the measuring piston adversely affect the accuracy and repeatablilty of the measurements. When the fluid is gas, these adverse effects are exacerbated by the compressibility of gas. Compressibility also makes it difficult to control the pressure of a gas passing through the flowmeter under test.